The spark plug used in the combustion device such as the internal combustion engine or the like includes, for example, an insulator having an axial hole extending in an axial direction, a center electrode inserted into the axial hole, a metal shell assembled on an outer periphery of the insulator, and a rod-shaped ground electrode having one end fixed to a leading end of the metal shell. Also, a substantially center portion of the ground electrode is bent back, and a spark discharge gap is formed between a leading end of the center electrode and the other end of the ground electrode. When a high voltage is applied to the center electrode, the spark discharge is generated in the spark discharge gap to ignite an air-fuel mixture.
Incidentally, when the spark discharge gap is enlarged due to an electrode wear, or carbon is stuck onto a surface of the insulator, without the generation of a normal spark discharge in the spark discharge gap, there is a risk that a current flows from the center electrode to the metal shell through a surface of the insulator, or a flying spark is generated between the insulator and the metal shell.
Under the circumstances, in order to prevent electric discharge (irregular discharge) outside of the spark discharge gap, there has been proposed a technique of providing an annular space (so-called “thermo clearance”) formed between an outer peripheral surface of the leading end side of the center electrode and an inner peripheral surface of the axial hole, and opened toward the leading end side (for example, refer to Patent reference 1). With the provision of the annular space, a distance from the center electrode to the metal shell along the surface of the insulator, or a distance between the center electrode and the leading end of the insulator can be relatively increased, and the generation of the irregular discharge can be more surely suppressed.